Theoretical Study Of Singlet Fission Dynamics In Organic Molecular Aggregates

Energy is the core issue of human development.Fossil fuels,such as coal,oil,natu?ral gas and recently discovered shale gas,are thought to be the most economical sources of energy.Fossil fuels,however,are limited resources.With the decrease of reserves,the cost will increase dramatically;and long-term usage of fossil fuels can cause en-vironmental problems such as acid rain and greenhouse effect.So there is an urgent need to find renewable sources of clean energy that can replace fossil fuels.Started form the time that the lives were born on earth,they use the thermal radiation from the sun to live.Compared with the human history,solar energy is an inexhaustible reser-voir of energy,using the solar energy to generate electricity is an important renewable and clean energy technology and is worthy of effort to develop.Solar cells are the key components of solar power generation.In many kinds of solar cells,the organic solar cells can be easily prepared and massively produced.Besides,they are flexible.With these advantages,these kinds of solar cells have received a lot of attention.However,organic solar cells have the disadvantage of low energy conversion efficiency,which limits their application.Singlet Fission(SF)in specific organic materials can convert the singlet exciton produced by light excitation into a pair of triplet excitons,in principle,it can realize the quantum efficiency over 100%,this can help traditional solar cell breakthrough en-ergy conversion efficiency limit.In order to further develop the practical application of singlet fission effect,there has been a lot of theoretical and experimental researches re-cently.However,there is still a lot of controversy on the singlet fission mechanism and the corresponding molecular aggregation effect,in addition.how the triplet pair state is separated to independent triplet excitons is also not clear.In this thesis,from the the-oretical aspect,the effects of exciton delocalization and quantum interference between pathways on singlet fission rate are investigated based on electronic structure calcula-tion method and quantum dynamic simulation method.Through numerical simulations and calculations of the realistic molecular systems,we provide some theoretical guid-ance for designing highly efficient singlet fission materials.The specific content of this thesis consists of the following six chapters:The first chapter is the introduction,which introduces the whole background.First of all,we introduced the common solar cells,including organic solar cells,dye/quantum dot sensitized solar cells and perovskite solar cells,and the working principles of them.After that,we introduced the Shockley-Queisser limit of energy conversion efficiency of solar cells,including its basic theoretical hypothesis and derivation processes.Then,the types of solar cells that break through the traditional efficiency limit are introduced,in addition with the principle of increasing photoelectric energy conversion efficiency by utilizing the properties of multiple exciton generation.Finally,the conditions and mechanism singlet fission are introduced,including the current research status of the corresponding area.The second chapter introduces the theoretical model and calculation method adopted in this thesis.Mainly including quantum chemistry and quantum dynamics simulation method,and the parameter calculation method for the diabatic model.For the quan-tum chemistry method,start with the Schrodinger equation,the Hartree-Fock method based on wave function is introduced under the Born-Oppenheimer approximation.In addition,the density functional theory based of electron density and the corresponding Kohn-Sham equation and the exchange-correlation functional form are provided.For the molecular aggregate model,the common methods for calculation of diabatic state energy(site energy),electronic coupling and spectral density are introduced.Then the quantum dynamics simulation method is introduced,first of all,the full quantum dy-namic and mixed quantum-classical dynamic methods are introduced;and then,we give the detailed derivation process of the time-dependent wave packet diffusion method.Fi-nally,this chapter introduces the quantum chemistry computing software packages used in this thesis.In the third chapter,we use the time-dependent wavepacket diffusion method(TD-WPD),which can account for the electron-phonon coupling effect,to study the effects of exciton delocalization and charge transfer(CT)state on singlet fission efficiency.By adjusting the parameters in the exciton model including the charge transfer state,the mechanism of how singlet fission is affected by the various of factors is analyzed.The results show that the intermediate charge transfer state has an important influence on the singlet fission process.Furthermore,the quantum interference effect between direct and indirect pathways,the latter is mediated by the charge transfer states,can be clearly ob-served.For the case that only considering the direct interaction between singlet excited states and triplet pair states,it is found that the exciton-exciton coupling between singlet excited states can make the singlet exciton population transfer spatially,besides,it can also make the singlet exciton spatially coherent with each other.The competitive result of these two effects leads to an optimal exciton-exciton coupling value,at which the sin-glet fission has a maximum rate.At the same time,it is found that the coupling between triplet pair states can slow down the singlet fission process.The main significance of this study is to give a physical image of the influence of the exciton delocalization on the singlet fission process.In the forth chapter,the quantum interference effect in singlet fission is studied.Through theoretical analysis,it is found that the coupling sign between singlet excitons can have a significant influence on quantum interference in the singlet fission process.This is similar to the effects of J-/H-aggregates that influence the aggregate emission efficiency.But the difference is that the quantum interference in singlet fission is addi-tionally dependent on the coupling values between singlet exciton and triplet pair states.We carefully studied the mathematical principles behind this physical phenomenon.In this study,the traditional J-/H-aggregate concept is introduced into the understanding of singlet fission process,it is of great significance from theoretical aspect,also,it has certain guiding significance to the realistic material design.Chapter five is the investigation of quantum interference in singlet fission of pen-tacene molecular aggregates.Using the electronic structure method to construct effec-tive model Hamiltonian,as well as the time-dependent wavepacket diffusion method for quantum simulation,the quantum interference behavior in the singlet fission is studied.Numerical simulations show that the quantum interference effect in singlet fission in pentancene can be adjusted via tuning the aggregate morphology.This study is based on the calculation of the realistic molecular system,it can be used to guide and design materials with good singlet fission property.Chapter six is the summary of this thesis,and the existing problems and future development direction of singlet fission process are also analyzed.